In the manufacture of board products, and particularly paperboard products such as corrugated paperboard, it is desirable to measure various properties of the paperboard product during manufacture, for example for the purposes of quality control. It is especially useful to measure “damage” or degradation, which may occur during manufacture of paperboard. It is generally accepted that degradation may usefully be assessed by measuring the strength of the finished board products.
Shear stiffness, and in particularly machine direction (MD) shear, is one property that provides an indication of paperboard performance.
One known method for measuring shear stiffness is by a twisting test, as disclosed in U.S. Pat. No. 4,958,522. However, this method has a number of disadvantages. Firstly, it requires that samples be prepared for testing, which must be accurately cut to size from the manufactured paperboard. The test apparatus is relatively bulky, complex and expensive. Furthermore, since it operates upon cut samples, the twisting test is a destructive test that cannot be used for continuous on-line measurement of board products during manufacture. The lack of a practical continuous measurement method results in uncertainty in the strength of manufactured board products, and accordingly it has become the usual practice to manufacture board products using heavier grade materials in order to provide a sufficient strength margin to compensate for this uncertainty.
It is therefore considered desirable to provide a method of assessing relevant properties of paperboard that may be employed on-line during manufacture, or for non-destructive random or spot testing of manufactured board products, for purposes of quality control, thereby enabling the use of cheaper, lighter grade materials, due to a reduced need to allow for a strength margin in the finished board products.
A non-destructive test method and apparatus is disclosed in international publication no. WO 01/53828 (equivalent to US publication no. 2003/0136199). The apparatus and method disclosed therein have the advantage that the sample to be tested does not need to be cut, thereby reducing or eliminating the preparation time. A predetermined force is applied to a corrugated board product, such that the board is not deflected beyond an elastic region. According to the disclosure, the force is applied via a plunger, and the deflection is limited by a self-limiting apparatus ensuring that the board remains exercised within its elastic limit. The resulting force-displacement characteristic is used to provide indicative results of MD shear by calculating the gradient of the curve within the elastic region.
However, the disclosure of WO 01/53828 admits at least the following problems with the elastic deflection method. A discrepancy was identified between MD shear and the force-displacement characteristic depending upon the board grade. It is therefore necessary to calibrate the apparatus according to differing grades of board being measured, since the measurement result is dependent upon the weight of materials, and not solely upon intrinsic characteristics of the paperboard. Additionally, results show a strong dependency on the velocity of the moving board when on-line measurements are performed. This requires the measured results to be corrected according to a logarithmic relationship. Overall, therefore, the elastic deflection method requires additional information to be determined both prior to and during the measurement, in order to ensure that appropriate corrections are made to the measured result.
A need therefore remains for an improved method and apparatus for measuring the properties of board products, which is compatible with performing continuous non-destructive on-line measurement, and which can provide at least a useful alternative to prior art measurement techniques, preferably also mitigating the aforementioned known problems in the art. It would also be desirable to provide a compact, portable, apparatus for testing of board products, which may be employed for quality control purposes, such as in random or spot testing of manufactured products.
It will be appreciated that the foregoing discussion is provided to explain the context of the invention, and should not be taken as an admission that any of this material formed part of the prior art base, or of the common general knowledge, in the relevant art on or before the priority date of any of the statements herein, or any claims appended hereto.